Spread spectrum base station notch filtering transmitted signals

ABSTRACT

A spread spectrum base station generates a plurality of spread spectrum signals. The spread spectrum signals encompass a selected frequency spectrum. Frequencies within the selected frequency spectrum having a high microwave power are detected. The spread spectrum signals are notch filtered at the detected frequencies. The notch filtered spread spectrum signals are transmitted.

[0001] This application is a continuation of U.S. patent applicationSer. No. 09/846,068, filed May 1,2001, which is a continuation of U.S.patent application Ser. No. 09/602,718, filed Jun. 26, 2000, now U.S.Pat. No. 6,243,370, which is a continuation of U.S. patent applicationNo. 08/272,498, filed Jan. 21, 1994, now U.S. Pat. No. 6,115,368, whichis a file wrapper continuation of U.S. patent application Ser. No.08/015,574, filed Feb. 5, 1993, which is a continuation of U.S. patentapplication Ser. No. 07/700,788, filed May 15, 1991, now U.S. Pat. No.5,185,762, which applications are incorporated herein by reference.

BACKGROUND

[0002] This invention relates to spread spectrum communications and moreparticularly to a personal communications network which communicatesover the same spectrum as used by a plurality of existing narrowbandmicrowave users.

DESCRIPTION OF THE PRIOR ART

[0003] The current fixed service, microwave system uses the frequencyband 1.85-1.99 GHz. Microwave users in this frequency band typicallyhave a bandwidth of 10 MHz or less.

[0004] A problem in the prior art is the limited capacity of thechannel, due to the number of channels available in the fixed service,microwave system.

SUMMARY

[0005] A spread spectrum base station generates a plurality of spreadspectrum signals. The spread spectrum signals encompass a selectedfrequency spectrum. Frequencies within the selected frequency spectrumhaving a high microwave power are detected. The spread spectrum signalsare notch filtered at the detected frequencies. The notch filteredspread spectrum signals are transmitted.

BRIEF DESCRIPTION OF THE DRAWING(S)

[0006] The accompanying drawings, which are incorporated in andconstitute a part of the specification, illustrate preferred embodimentsof the invention, and together with the description serve to explain theprinciples of the invention.

[0007]FIG. 1 is a block diagram of a PCN-base station receiver;

[0008]FIG. 2A is a block diagram of a first embodiment of a PCN-basestation transmitter;

[0009]FIG. 2B is a block diagram of a second embodiment of a PCN-basestation transmitter;

[0010]FIG. 2C is a detailed block diagram of a transmitter;

[0011]FIG. 3 is a block diagram of a PCN-unit receiver;

[0012]FIG. 4A is a block diagram of a first embodiment of PCN-unittransmitter;

[0013]FIG. 4B is a block diagram of a second embodiment of a PCN unittransmitter;

[0014]FIG. 4C is a detailed block diagram of a PCN transmitter;

[0015]FIG. 5 shows the spectrum of a spread spectrum signal with an AMsignal of equal power at its carrier frequency;

[0016]FIG. 6 shows a spread spectrum data signal when the spreadspectrum signal power is equal to an AM signal power;

[0017]FIG. 7 shows an audio signal when the spread spectrum signal poweris equal to the AM signal power;

[0018]FIG. 8 shows a possible pseudo-random sequence generator;

[0019]FIG. 9 shows possible position settings of switches of FIG. 8 toform PN sequences;

[0020]FIG. 10 illustrates a PCN system geographic architecture accordingto the present invention;

[0021]FIG. 11 shows fixed service microwave and PCN user geometry andurban propagation models;

[0022]FIG. 12 illustrates a typical fixed service microwave user antennapattern versus elevation angle;

[0023]FIG. 13 shows link parameters for a typical 2 GHz fixed servicelink and a typical PCN system;

[0024]FIG. 14 illustrates calculated in-band received power at a fixedservice microwave receiver in the presence of PCN users;

[0025]FIG. 15 depicts the region where a PCN handset has an error rate,P_(e)>10², due to fixed service microwave transmission and PCN versushandset cell range;

[0026]FIG. 16 shows a PCN field test experiment;

[0027] FIGS. 17A-17K show measured attenuation versus distance; and

[0028]FIG. 18 shows the spectrum of a spread spectrum signal withmultipath.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0029] Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings, wherein like reference numerals indicate likeelements throughout the several views.

[0030] This patent is related to U.S. patent application having Ser. No.07/622,235, filing date Dec. 5, 1990, now U.S. Pat. No. 5,351,269, andto U.S. patent application having Ser. No. 07/626,109, filing date Dec.14,1990, now U.S. Pat. No. 5,228,056, both by Donald L. Schilling, whichare both incorporated herein by reference.

[0031] The spread spectrum CDMA communications system of the presentinvention is located within a same geographical region as occupied by atleast one fixed service, microwave system or other microwave system.Each fixed service microwave system communicates over a fixed-servicemicrowave channel, which has a fixed-service bandwidth. In presentlydeployed fixed service microwave systems, the fixed-service bandwidth is10 MHz or less.

[0032] In the 1.85-1.99 GHz region, the spectrum is used by a pluralityof narrowband users, with each microwave user using one of a pluralityof fixed-service-microwave channels. A first fixed-service microwavesystem using a first fixed-service microwave channel is separated infrequency by a guard band from a second fixed-service microwave systemusing a second fixed-service microwave channel. The first fixed-servicemicrowave system may be separated geographically or spatially from thesecond fixed-service microwave system.

[0033] The spread spectrum CDMA communications system, which uses directsequence (DS) spread spectrum modulation includes a plurality ofPCN-base stations and a plurality of PCN units located within the samegeographical region as occupied by the plurality of microwave users ofthe fixed service microwave system. The spread spectrum CDMAcommunications system can be used for communicating data between aplurality of PCN users. The data may be, but are not limited to,computer data, facsimile data or digitized voice.

[0034] A PCN-base station, which typically is not collocatedgeographically with a fixed service microwave station, communicates databetween a plurality of PCN users. A first PCN user uses a first PCNunit, and a second PCN user uses a second PCN unit, etc.

[0035] Each PCN-base station includes base-converting means,base-product-processing means, base-transmitting means, base-detectionmeans and a base antenna. Each base station optionally may havebase-filter means. The base-detection means may includebase-spread-spectrum-processing means and base-synchronizing means. Thebase-detection means broadly is a repeater which converts spreadspectrum coded data communicated from one PCN unit into a form suitablefor another PCN user or telecommunication user.

[0036] Each of the PCN-base stations may be geographically spaced suchthat the power radiated by the base-transmitting means from within itscell up to a contiguous cell of a neighboring PCN-base station primarilyvaries inversely with distance by an exponent of approximately two, andthe power radiated by the base-transmitting means outside its cellprimarily varies inversely with distance by an exponent which is greaterthan two, typically four or more.

[0037] The geographic spacing of cells typically is small, on the orderof 1200 to 2000 feet. The small spacing allows the use of lowtransmitter power, so as not to cause interference with thefixed-service microwave systems. Also, an in-band fixed servicemicrowave user is often spatially and geographically distant from thePCN system, and when this occurs it results in negligible interferencewith the fixed-service microwave user. As set forth below, base-filtermeans inserts notches in the power spectrum transmitted from thebase-transmitting means, which essentially eliminates all interferencefrom the base station to a fixed-service microwave system.

[0038] The base-spread-spectrum-processing means, as illustrated in FIG.1, may be embodied as a pseudorandom generator, a plurality of productdevices 141 and a plurality of bandpass filters 143. The pseudorandomgenerator stores chip codes, g₁ (t), g₂ (t), . . . , g_(N) (t), fordemodulating data from spread spectrum signals received from theplurality of PCN units at the PCN-base station. The base-detection meansalso includes means for synchronizing thebase-spread-spectrum-processing means to received spread spectrumsignals.

[0039] The base-spread-spectrum-processing means at the PCN-base stationprocesses selected data received from a selected PCN unit, which weretransmitted with a spread spectrum signal using a selected-chip code,g_(i) (t). The detector 145 demodulates the selected data from thedespread spread-spectrum signal.

[0040] A plurality of product devices 141, bandpass filters 143 anddetectors 145 may be coupled through a power splitter 147 to an antenna149, for receiving simultaneously multiple spread-spectrum channels.Each product device 141 would use a selected chip code for demodulatinga selected spread spectrum signal, respectively.

[0041] For a spread spectrum system to operate properly, the spreadspectrum receiver must acquire the correct phase position of thereceived spread spectrum signal, and the receiver must continually trackthat phase position so that loss-of-lock will not occur. The twoprocesses of acquisition and tracking form the synchronization subsystemof a spread spectrum receiver. The former operation is typicallyaccomplished by a search of as many phase positions as necessary untilone is found which results in a large correlation between the phase ofthe incoming signal and the phase of the locally generated spreadingsequence at the receiver. This former process occurs using correlatormeans or matched filter means. The latter tracking operation is oftenperformed with a “delay-locked loop”. The importance of the combinedsynchronization process cannot be overstated for if synchronization isnot both achieved and maintained, the desired signal cannot be despread.

[0042] The base-converting means, as illustrated in FIG. 2A, may beembodied as a plurality of base modulators 151. A base modulator 151converts the format of data to be transmitted to a PCN user into a formsuitable for communicating over radio waves. For example, an analogvoice signal may be converted to a base-data signal, using a techniquecalled source encoding. Typical source coders are linear predictivecoders, vocoders, delta modulators and pulse code modulation coders.

[0043] The base-product-processing means may be embodied as a pluralityof base-spread-spectrum modulators 153. A base-spread-spectrum modulator153 is coupled to a base modulator 151. The base-spread-spectrummodulator 153 modulates the converted-data signal using spread spectrum.The converted data is multiplied using a product device or modulo-2added, using an EXCLUSIVE-OR gate 153 with a selected spread-spectrumchip code, g_(N+i) (t), as shown in FIG. 2B. The spread-spectrumbandwidth of the converted data is much greater than, at leastapproximately two times, the narrowband bandwidth of a fixed servicemicrowave user. The spread-spectrum bandwidth typically overlays inspectrum one or more fixed-service channels. In a preferred embodiment,the spread-spectrum bandwidth is 48 MHz.

[0044] The base-transmitting means may be embodied as a plurality ofbase transmitters 155. A base transmitter 155 is coupled to abase-spread-spectrum modulator 153. The base transmitter 155 transmitsacross the fixed service microwave bandwidth, thespread-spectrum-processed-converted data from the PCN-base station to aPCN unit. The base transmitter 155 includes modulating the spreadspectrum processed converted data at a carrier frequency, f_(o).

[0045] The base-transmitter 155 has a transmitter oscillator whichsupplies a carrier signal at the carrier frequency. The transmitteroscillator is coupled to a transmitter product device. The transmittermultiplies, using the transmitter-product device, thespread-spectrum-processed-converted data by the carrier signal. A moredetailed description of transmitter 155 is provided in FIG. 2C.

[0046] The base-transmitting means may, in a preferred embodiment,transmit data using a spread spectrum signal having a power levellimited to a predetermined level. The base-transmitting means maytransmit data by adding the plurality of spread spectrum data signals.

[0047] A plurality of modulators 151, product devices 153 andtransmitters 155 may be coupled through a power combiner 157 to anantenna 159 for simultaneously transmitting a multiplicity ofspread-spectrum channels. FIG. 2A is an illustrative embodiment forgenerating simultaneous spread spectrum signals, and there are manyvariants for interconnecting product devices, modulators andtransmitters, for accomplishing the same function.

[0048] As an alternative example, FIG. 2B illustrates a PCN-base stationtransmitter which may be used for producing the same result as thetransmitter of FIG. 2A. In FIG. 2B data are modulo-2 added, usingEXCLUSIVE-OR gates 253 with a selected spread-spectrum chip code,g_(N+i) (t). The resulting spread-spectrum-processed data from aplurality of EXCLUSIVE-OR gates 253 are combined using combiner 257. Thebase transmitter 255 modulates the combined spread-spectrum-processeddata at the carrier frequency, f_(o). The transmitter 255 is coupled tothe antenna 159 and simultaneously transmits the plurality ofspread-spectrum-processed data as a multiplicity of spread-spectrumchannels.

[0049]FIG. 2C illustratively shows the base-filter means embodied as anotch filter 525, as part of a PCN-base transmitter. The embodimentshown in FIG. 2C may be employed in the base transmitters 155, 255 ofFIGS. 2A and 2B. The notch filter 525 inserts one or more notches in thepower spectrum transmitted from the base-transmitting means. The notchesare located at the same frequency as a fixed-service, microwave channel,and typically have the same bandwidth, a fixed-service bandwidth, as thefixed-service microwave channel. Preferably, the notches provide 15 dBor more attenuation.

[0050] The notch filter 525 can be implemented at an intermediatefrequency of the transmitter, or with technology permitting, the notchfilter could operate at the carrier frequency, f_(o). The notch filter525 is shown as an example, in FIG. 2C, coupled between a firsttransmitter mixer 522 and a second transmitter mixer 524. The firsttransmitter mixer 522 is coupled to a first local oscillator 521, andthe second transmitter mixer 524 is coupled to a second local oscillator523. A transmitter typically has a power amplifier 528 coupled to anoutput of the second local oscillator 523.

[0051] In FIG. 2C, the first local oscillator 521 supplies a signal tothe first transmitter mixer 522 for modulating the combinedspread-spectrum-processed data from combiner 257. The second localoscillator 523 supplies a signal to the transmitter mixer 524 formodulating the notched combined spread-spectrum-processed data,outputted from the notch filter 525, to the carrier frequency.

[0052] Assume that the bandwidth of the combinedspread-spectrum-processed data is much greater than that offixed-service microwave user's bandwidth. The notch filter 525 caninsert notches in the spectrum of the combined spread-spectrum-processeddata such that when the combined spread-spectrum-processed data aremodulated and transmitted at the carrier frequency, f_(o), the notchescoincide with the fixed-service, microwave channels.

[0053] Typically, a PCN-base station and the fixed-service microwavestation have fixed geographic locations, and the fixed-service microwavechannel is at a preassigned frequency and bandwidth. Thus, a notchfilter for a PCN-base station can be a fixed design. The notch filter ata PCN-base station alternatively may be an adjustable notch filter. Theadjustable notch filter can be responsive to a dynamic environment,where microwave signals or channels appear unexpectedly.

[0054] The notch in the spectrum of the transmitted spread-spectrumsignals from the PCN-base station is less than the bandwidth of thespectrum. For example, the transmitted spread-spectrum signals from aPCN-base station might have a bandwidth of 48 MHz. The fixed-service,microwave channel might have a fixed-service bandwidth of less than 10MHz. Thus, in this example, a notch filter would reduce the energy inthe transmitted spread-spectrum signal from the PCN-base station byapproximately only 20% or less.

[0055] The present invention also includes PCN units which are locatedwithin the cell. Each of the PCN units has a PCN antenna, PCN-detectionmeans, PCN-converting means, PCN-product-processing means, PCN-filtermeans and PCN-transmitting means. The PCN-detection means is coupled tothe PCN-antenna. The PCN-detection means includesPCN-spread-spectrum-processing means.

[0056] The PCN-detection means recovers data communicated to the PCNunit from the PCN-base station. The detection means also includes meansfor converting the format of the data into a form suitable for a user.The format may be, for example, computer data, an analog speech signalor other information. The PCN-detection means, by way of example, mayinclude tracking and acquisition circuits for the spread spectrumsignal, a product device for despreading the spread spectrum signal andan envelope detector. FIG. 3 illustratively shows an antenna 169 coupledto PCN detection means, which is embodied as a PCN spread-spectrumdemodulator 161, PCN-bandpass filter 163, and PCN-data detector 165.

[0057] The PCN-spread-spectrum demodulator 161 despreads, using achip-code signal having the same or selected chip code, g_(N+1) (t), asthe received spread-spectrum signal, the spread-spectrum signal receivedfrom the PCN-base station. The bandpass filter 163 filters the despreadsignal and the PCN-data detector 165 puts the format of the despreadspread-spectrum signal into a form suitable for a PCN user.

[0058] The PCN-spread-spectrum-processing means includes means forstoring a local chip code, g_(N+i) (t), for comparing to signalsreceived for recovering data sent from the PCN-base station to the PCNunit.

[0059] The PCN-spread-spectrum-processing means also may include meansfor synchronizing the PCN-spread-spectrum-processing means to receivedsignals. Similarly, the PCN-spread-spectrum-processing means at thePCN-base station includes means for processing data for particular PCNunits with a selected chip code.

[0060] The PCN-converting means, as illustrated in FIG. 4A, may beembodied as a PCN modulator 171. The PCN modulator 171 converts theformat of the data into a form suitable for communicating over radiowaves. Similar to the PCN-base station, an analog voice signal may beconverted to a converted-data signal, using a technique called sourceencoding. As with the base modulator 151, typical source encoders arelinear predictive coders, vocoders, adaptive delta modulators and pulsecode modulators.

[0061] The PCN-product-processing means may be embodied as aPCN-spread-spectrum modulator 173. The PCN-spread-spectrum modulator 173is coupled to the PCN modulator 171. The PCN-spread-spectrum modulator173 modulates the converted-data signal with a selected chip code, g_(i)(t). The converted-data signal is multiplied using a product device withthe selected chip code, g_(i) (t). The spread-spectrum bandwidth of theconverted data is much greater than, approximately five times greater inthe preferred embodiment, the narrowband bandwidth of a fixed servicemicrowave user. In a preferred embodiment, the spread-spectrum bandwidthis 48 MHz. The spread-spectrum bandwidth from the PCN modulator 171 isthe same as that from the modulator 151 at the PCN-base station, and mayoverlay the same microwave frequency or overlay separate microwavefrequencies.

[0062] As an equivalent transmitter, FIG. 4B illustrates a transmitterfor a PCN unit having PCN-spread-spectrum-processing means as a PCNmodulo-2 adder, embodied as an EXCLUSIVE-OR gate 273. The EXCLUSIVE-ORgate 273 modulo-2 adds the converted data signal with the selected chipcode, g_(i) (t).

[0063] The PCN-transmitting means in FIGS. 4A and 4B may be embodied asa PCN transmitter 175. The PCN transmitter 175 is coupled between thePCN-spread-spectrum modulator 173 and antenna 179. The PCN transmitter175 transmits across the fixed-service microwave bandwidth, thespread-spectrum-processed-converted data from the PCN unit to thePCN-base station. The PCN transmitter 175 modulates thespread-spectrum-processed-converted data at a carrier frequency, f_(o).The carrier frequency of the PCN transmitter and the cell transmittermay be at the same or at different frequencies. Typically the PCNtransmitter and the cell transmitter use the same frequency if halfduplex is used, and two frequencies if full duplex is used.

[0064] The PCN-filter means inserts one or more notches in the powerspectrum transmitted from the PCN-transmitting means. The notches arelocated at the same frequency as a fixed-service, microwave channel, andtypically have the same bandwidth as a fixed-service microwave channel.Preferably, the notches provide 15 dB or more attenuation.

[0065]FIG. 4C illustrates the PCN-filter means embodied preferably as anadjustable-notch filter 725 as part of a PCN transmitter. The embodimentshown in FIG. 4C may be employed in the PCN transmitter 175 of eitherFIG. 4A or FIG. 4B. The adjustable notch filter 725 is shown implementedat an intermediate frequency of the PCN transmitter 175, although withtechnology permitting, the adjustable notch filter 725 can be at thecarrier frequency, f_(o), of the PCN transmitter 175.

[0066] The adjustable-notch filter 725 is coupled between a firstPCN-transmitter mixer 722 and a second PCN-transmitter mixer 724. Thefirst PCN-transmitter mixer 722 is coupled to a first PCN-localoscillator 721, and the second PCN-transmitter mixer 724 is coupled to asecond PCN-local oscillator 723. A transmitter typically has a poweramplifier 728 coupled to an output of the second local oscillator 723.

[0067] In FIG. 4C the first PCN-local oscillator 721 provides a firstoscillator frequency signal to the first PCN-transmitter mixer 722. Thefirst PCN-transmitter mixer 722 modulates the spread-spectrum-processeddata to the PCN-transmitter intermediate frequency, f_(IF). The secondPCN-local oscillator 723 provides a second oscillator signal to thesecond PCN-transmitter mixer 724. The second PCN-transmitter mixer 724modulates the notched spread-spectrum-processed data to a carrierfrequency, f_(o).

[0068] Assume that the bandwidth of the spread-spectrum-processed datais much greater that of the fixed-service bandwidth. Theadjustable-notch filter 725 can insert notches in the spectrum of thespread-spectrum-processed data such that when thespread-spectrum-processed data are modulated and transmitted at thecarrier frequency, f_(o), the notches coincide with the fixed-servicemicrowave channels.

[0069] A PCN unit is assumed to roam within a geographic region of oneor more cells. Thus, the PCN unit, at different locations, may tend tointerfere with fixed-service, microwave channels at differentfrequencies. The adjustable-notch filter 725 has its center frequencyand bandwidth set so as to notch the power spectrum from the PCNtransmitter at whatever desired frequency and bandwidth of thefixed-service, microwave channel.

[0070] The adjustable-notch filter 725 can be controlled several ways.First, each PCN-base station can be programmed with the frequency andbandwidth of each fixed-service microwave user which transmits acrossthe geographic region of the base station. The PCN-base station can senda command signal to the PCN unit through one of the spread-spectrumchannels, indicating which portions of spectrum to notch out with theadjustable-notch filter 725. A controller 726, which receives thecommand signal, can set the adjustable-notch filter 725 to one or morecenter frequencies and bandwidths. Note that this scenario assumes thatthe base station has knowledge of the location in frequency andbandwidths of the fixed-service, microwave channels operating within thesame geographic region of the cell.

[0071] Second, the PCN unit, or the PCN-base station, alternatively orin addition, may have a sensor which detects the microwave power orenergy of the one or more fixed-service, microwave channels. The sensordetermines the center frequency and the bandwidth of the fixed-servicemicrowave channel, and then the controller 726 adjusts theadjustable-notch filter 725 to notch the spread-spectrum-processed dataat those frequencies and bandwidths.

[0072] In a preferred embodiment the adjustable-notch filer 726 may beembodied as an adaptive transversal filter. Any tunable notch filter,however, can be used for the adjustable-notch filter 726. Asingle-tuned, resistor-inductor-capactor, RLC, circuit has been found tosuffice for many applications. The circuit may be tuned with a variablecapacator, i.e., a varicap controlled by a voltage.

[0073] The spread spectrum signals of the present invention are designedto be “transparent” to other users, i.e., spread spectrum signals aredesigned to provide “negligible” interference to the communication ofother, existing users. The presence of a spread spectrum signal isdifficult to determine. This characteristic is known as low probabilityof interception (LPI) and low probability of detection (LPD). The LPIand LPD features of spread spectrum allow transmission between users ofa spread spectrum CDMA communications system without the existing usersof the mobile cellular system experiencing significant interference. Thepresent invention makes use of LPI and LPD with respect to thepredetermined channels in the fixed-service microwave system. By havingthe power level of each spread spectrum signal below the predeterminedlevel, then the total power from all spread spectrum users within a celldoes not interfere with microwave users in the fixed-service microwavesystem.

[0074] The PCN units and optionally the base stations can have a notchfilter in their respective transmitters for reducing the powertransmitted from the PCN unit and base station at the frequency andfixed-service bandwidth of the fixed-service microwave system.Accordingly, the PCN system, as disclosed herein, can overlay an alreadyexisting fixed-service microwave system without causing any interferenceto the fixed-service microwave system. The effect on the PCN system ofnotching a portion of the bandwidth of the spread spectrum signal isminimal inasmuch as the notch removes only a small portion of the totalpower in the spectrum of the spread spectrum signal. It has been foundexperimentally that the use of such filters does not noticeably affectthe acquisition time or the tracking capability of the system. Indeed,no deleterious affects were observed.

[0075] Spread spectrum is also “jam” or interference resistant. A spreadspectrum receiver spreads the spectrum of the interfering signal. Thisreduces the interference from the interfering signal so that it does notnoticeably degrade performance of the spread spectrum system. Thisfeature of interference reduction makes spread spectrum useful forcommercial communications, i.e., the spread spectrum waveforms can beoverlaid on top of existing narrowband signals. Accordingly, signalsfrom an already existing fixed-service microwave system cause negligibledegradation in performance of the spread-spectrum system.

[0076] The present invention employs direct sequence spread spectrum,which uses a phase (amplitude) modulation technique. Direct sequencespread spectrum takes the power that is to be transmitted and spreads itover a very wide bandwidth so that the power per unit bandwidth(watts/hertz) is minimized. When this is accomplished, the transmittedspread spectrum power received by a microwave user, having a relativelynarrow bandwidth, is only a small fraction of the actual transmittedpower.

[0077] In a fixed-service microwave system, by way of example, if aspread spectrum signal having a power of 1 milliwatt is spread over afixed-service microwave bandwidth of 48 MHz and a microwave user employsa communication system having a channel bandwidth of only 10 MHz, thenthe effective interfering power due to one spread spectrum signal, inthe narrow band communication system, is reduced by the factor of 48MHz/10 MHz which is approximately 5. Thus, the effective interferingpower is 1 milliwatt (mW) divided by 5 or 0.2 mW. For fifty concurrentusers of spread spectrum, the power of the interfering signal due tospread spectrum is increased by fifty to a peak interfering power of 10mW.

[0078] The feature of spread spectrum that results in interferencereduction is that the spread spectrum receiver actually spreads thereceived energy of any interferer over the same wide bandwidth, 50 MHzin the present example, while compressing the bandwidth of the desiredreceived signal to its original bandwidth. For example, if the originalbandwidth of the desired PCN data signal is only 30 kHz, then the powerof the interfering signal produced by the cellular base station isreduced by 50 MHz/30 kHz which is approximately 1600.

[0079] Direct sequence spread spectrum achieves a spreading of thespectrum by modulating the original signal with a very wideband signalrelative to the data bandwidth. This wideband signal is chosen to havetwo possible amplitudes, +1 and −1, and these amplitudes are switched,in a “pseudo-random” manner, periodically. Thus, at each equally spacedtime interval, a decision is made as to whether the wideband modulatingsignal should be +1 or −1. If a coin were tossed to make such adecision, the resulting sequence would be truly random. However, in sucha case, the receiver would not know the sequence a-priori and could notproperly receive the transmission. Instead a chip-code generatorgenerates electronically an approximately random sequence, called apseudo-random sequence, which is known a-priori to the transmitter andreceiver.

[0080] To illustrate the characteristics of spread spectrum, consider4800 bps data which are binary phase-shift keyed (BPSK) modulated. Theresulting signal bandwidth is approximately 9.6 kHz. This bandwidth isthen spread using direct sequence spread espectrum to 16 MHz. Thus, theprocessing gain, N, is approximately 1600 or 32 dB.

[0081] Alternatively, consider a more typical implementation with 4800bps data which is modulo-2 added to a spread-spectrum-chip-code signal,g_(i) (t), having a chip rate of 25 Mchips/sec. The resultingspread-spectrum data are binary-phase-shift keyed (BPS K) modulated. Theresulting spread-spectrum bandwidth is 50 MHz. Thus, the processing gainis: N′=(25×10⁶)/(4.8×10³), which approximately equals 5000, or 37 dB.

[0082]FIG. 5 shows the spectrum of this spread spectrum signal of anamplitude modulated 3 kHz sinusoidal signal, when they each have thesame power level. The bandwidth of the AM waveform is 6 kHz. Bothwaveforms have the same carrier frequency.

[0083]FIG. 6 shows the demodulated square-wave data stream. Thiswaveform has been processed by an “integrator” in the receiver, hencethe triangular shaped waveform. Note that positive and negative peakvoltages representing a 1-bit and 0-bit are clearly shown. FIG. 7 showsthat the demodulated AM signal replicates the 3 kHz sine wave.

[0084] The AM signal does not degrade the reception of data because thespread spectrum receiver spreads the energy of the AM signal over 16MHz, while compressing the spread spectrum signal back to its original9.6 kHz bandwidth. The amount of the spread AM energy in the 9.6 kHzBPSK bandwidth is the original energy divided by N=1600 (or,equivalently, it is reduced by 32 dB). Since both waveforms initiallywere of equal power, the signal-to-noise ratio is now 32 dB, which issufficient to obtain a very low error rate.

[0085] The spread spectrum signal does not interfere with the AMwaveform because the spread spectrum power in the bandwidth of the AMsignal is the original power in the spread spectrum signal divided byN₁, where$N_{1} = {\frac{16\quad {MHz}}{6\quad {kHz}} = {2670\quad \left( {{or}\quad 33\quad {dB}} \right)}}$

[0086] hence the signal-to-interference ratio of the demodulated sinewave is 33 dB.

[0087] The direct sequence modes of spread spectrum uses psuedo randomsequences to generate the spreading sequence. While there are manydifferent possible sequences, the most commonly used are“maximal-length” linear shift register sequences, often referred to aspseudo noise (PN) sequences. FIG. 8 shows a typical shift registersequence generator. FIG. 9 indicates the position of each switch bi toform a PN sequence of length L, where

L=2^(n)−1

[0088] The characteristics of these sequences are indeed “noise like”.To see this, if the spreading sequence is properly designed, it willhave many of the randomness properties of a fair coin toss experimentwhere “1”=heads and “−1”=tails. These properties include the following:

[0089] 1) In a long sequence, about ½ the chips will be +1 and ½ will be−1.

[0090] 2) The length of a run of r chips of the same sign will occurabout L/2^(r) times in a sequence of L chips.

[0091] 3) The autocorrelation of the sequence PN_(i) (t) and PN_(i)(t+τ) is very small except in the vicinity of τ=0.

[0092] 4) The cross-correlation of any two sequences PN_(i) (t) andPN_(j) (t+τ) is small.

[0093] Code Division Multiple Access

[0094] Code division multiple access (CDMA) is a direct sequence spreadspectrum system in which a number, at least two, of spread-spectrumsignals communicate simultaneously, each operating over the samefrequency band. In a CDMA system, each user is given a distinct chipcode. This chip code identifies the user. For example, if a first userhas a first chip code, g₁ (t), and a second user a second chip code, g₂(t), etc., then a receiver, desiring to listen to the first user,receives at its antenna all of the energy sent by all of the users.

[0095] However, after despreading the first user's signal, the receiveroutputs all the energy of the first user but only a small fraction ofthe energies sent by the second, third, etc., users.

[0096] CDMA is interference limited. That is, the number of users thatcan use the same spectrum and still have acceptable performance isdetermined by the total interference power that all of the users, takenas a whole, generate in the receiver. Unless one takes great care inpower control, those CDMA transmitters which are close to the receiverwill cause the overwhelming interference. This effect is known as the“near-far” problem. In a mobile environment the near-far problem couldbe the dominant effect. Controlling the power of each individual mobileuser is possible so that the received power from each mobile user is thesame. This technique is called “adaptive power control”. See U.S. PatentApplication having Filing Date of Nov. 16, 1990, entitled, “AdaptivePower Control Receiver,” by Donald L. Schilling, which is incorporatedherein by reference.

[0097] The Proposed PCN Spread Spectrum CDMA System

[0098] The PCN spread spectrum communications system of this patent is aCDMA system. Direct Sequence Code Division Multiple Access (CDMA) cansignificantly increase the use of spectrum. With CDMA, each user in acell uses the same frequency band. However, each PCN CDMA signal has aseparate pseudo random code which enables a receiver to distinguish adesired signal from the remaining signals. PCN users in adjacent cellsuse the same frequency band and the same bandwidth, and therefore“interfere” with one another. A received signal may appear somewhatnoisier as the number of users' signals received by a PCN base stationincreases.

[0099] Each unwanted user's signal generates some interfering powerwhose magnitude depends on the processing gain. PCN users in adjacentcells increase the expected interfering energy compared to PCN userswithin a particular cell by about 50%, assuming that the PCN users areuniformly distributed throughout the adjacent cells. Since theinterference increase factor is not severe, frequency reuse is notemployed. Each spread spectrum cell can use a full 48 MHz band fortransmission and a full 48 MHz band for reception. Hence, using a chiprate of twenty five million chips per second and a coding data rate of32 k bps results in approximately a processing gain of 750 chips perbit. It is well known to those skilled in the art that the number of PCNCDMA users is approximately equal to the processing gain. Thus, up to750 users can operate in the 50 MHz bandwidth overlaying one or morefixed service microwave systems in the 1.85-1.99 GHz region.

[0100] Shared Spectrum Capability of CDMA PCN

[0101] An interesting aspect of the use of DS CDMA for cellular radiotransmission is in the possibility of overlaying the DS CDMA PCN radionetwork on top of existing users occupying the frequency band ofinterest. That is, it is not necessary to supply to the spread spectrumusers a frequency band which is completely devoid of other users.Rather, if the frequency band is partially occupied by variousnarrowband users, it is often possible to superimpose the DS CDMAsignals on the same band in such a manner that both sets of users canco-exist.

[0102] A proposed PCN system geographic architecture is shown in FIG.10. A multiplicity of microcells each having a PCN-base station,communicate with a plurality of PCN users.

[0103] To see that CDMA PCN can coexist with fixed service (FS)microwave users, the effect of the mobile PCN users on the FS microwavereceiver and the effect of the FS microwave transmitter on a mobile PCNuser must be examined.

[0104] Effect of PCN users on a FS Microwave Receiver

[0105] To examine the effect of the mobile PCN user on a FS microwavereceiver, refer to FIG. 11. A PCN user is shown whose transmission isreceived by a microwave receiver. The PCN user's signal is attenuated by(1) path loss and (2) antenna directivity which results in a significantdecrease in the FS microwave antenna gain, FIG. 12, in the direction ofthe PCN user.

[0106] For example, the link parameters for a typical 2 GHz FS link andfor a PCN system are given in FIG. 13. The free space propagation loss,L_(uW), between FS transmitter and receiver is

L _(uW)=103+20 log(R),dB  (1)

[0107] while the path loss L_(PCN) between a PCN user and FS receivertypically is not the free space path loss as it is affected bymultipath. A standard representation, approved by the CCIR is:

L _(PCN)=135.5+33.21 log(d),dB

[0108] In these equations R is the distance, in miles, betweentransmitter and receiver and d is the distance, in miles, between PCNand receiver, see FIG. 11.

[0109] Using equations (1) and (2) and FIG. 13, the ratio of thereceived signal power P_(s) from the FS microwave transmitter to thereceived interference P_(I) of the PCN user(s) can be determined and isgiven the FIG. 14. In FIG. 14 it is assumed that multiple PCN users areall congregated at the same location, clearly a worst-case result. Italso should be noted that if P_(s)/P_(I)=23 dB the probability of asymbol being in error before FEC decoding is 10⁻³. The coding gain of atypical FS microwave receiver is 3 dB.

[0110] Assuming that there are 100 active PCN users/cell, uniformlydistributed across the cell, and there are 32 (or more) cells facing theFS microwave receiver, then the resulting P_(S)/P_(I)=53 dB, whichprovides a signal to noise ratio of 23 dB with a 30 dB fade margin. Thiscorresponds to an undecoded error rate of 10⁻³.

[0111] The addition of the notch filter at the PCN unit and/or PCN basestation significally reduces these already low values such thatinterference with a fixed-service microwave user is negligible or nonexistent. When the spread-spectrum system overlaps the antenna bean andis near the receiver of the fixed-service microwave system the notchfilter provides more than 15 dB additional attenuation to thespread-spectrum signal power in the band of the notch. When the totalpower of the spread-spectrum system is spread over 48 MHz and the FSbandwidth is less than 10 MHz, only 20% of the spread-spectrum power isavailable to interfere with the FS microwave system. Since most of thetime the spread-spectrum PCN-base station and PCN unit are at a remotedistance from a fixed-service microwave station, i.e. a fixed-servicemicrowave station is located outside the normal geographic coverage areaof a cell, the path loss from the PCN-base station or PCN user varies atan exponent greater than two, and typically by the fourth power. Also,most of the time a PCN-base station and PCN user are not operatingwithin the antenna beam of a fixed-service microwave station. Thus, thepower of the spread-spectrum signal at the fixed-service microwave useris reduced by 20 dB to 40 dB.

[0112] Effect of a FS Microwave Transmitter on a PCN User

[0113] To calculate the effect of the FS microwave transmitter on PCNusers, assume that there are 100 users uniformly distributed throughouteach cell and consider those cells “facing” the microwave transmitter.FIG. 15 shows the region where the bit error rate, before FEC decoding,exceeds 10-2. The dimensions of each cell are 1200 feet by 1200 feet.The area shown, therefore constitutes approximately 2.2% of the cellarea. Hence 2 to 3 users will be inconvenienced within that single cell.No users will be inconvenienced outside the region shown.

[0114] PCN Field Test

[0115] Experiments were conducted in the frequency band 1850-1990 MHz,to conduct field tests of a PCN system employing direct sequence spreadspectrum CDMA. The novel application seen here is that the band chosenfor experimentation is one which is used today for microwavetransmission. The field tests are intended to verify that spreadspectrum can share a band with existing users and thereby increase theutilization efficiency of a frequency band. These tests also provideimportant quantitative information, such as how many CDMA users and atwhat power level, can operate in the vicinity of a microwave receiverwithout degrading the microwave user's performance, and how many CDMAusers can operate in the vicinity of a microwave transmitter before theCDMA user's performance is degraded.

[0116] The field tests fall into two categories: measurement of theinterference produced by the spread spectrum PCN on the existingmicrowave users, and measurement of the interference produced by theexisting microwave users on both the mobile user and the cell. Theseexperiments were performed in New York and Orlando, Fla. during 1990 and1991.

[0117]FIG. 16 shows a typical, fixed location, existing microwavetransmitter-receiver site. The mobile users 1 and 2 each transmit to thecell using the frequency band 1860-1910 MHz and receive from the cellusing the band 1930-1980 MHz. Mobile user 3-50 is a transmit-only userwhich simulates 48 users transmitting from the same site. The powerlevel of mobile users 1 and 2 is adjustable from 100 uW to 100 mW, thepower level of mobile users 3-50 will be adjustable from 4.8 mW to 4.8W, and the power level of the cell is adjustable from 5 mW to 5 W. Eachadjustment is made independently of the others. Each mobile user had anotch filer located at the frequency and with a fixed-service microwavebandwidth of a fixed-service microwave user.

[0118] Measurement 1:

[0119] Measurement of the Interference Product by PCN on ExistingMicrowave Receiver

[0120] The four vans, which include the mobile users as well as thecell, shown in FIG. 16, were moved relative to a microwave receiver, andthe bit error rate (B ER) measured at each position. The measured BERsare compared to the interference-free BER obtained when the mobilesystem is off. Different transmit powers from the cell and from themobile users are employed in order to determine the robustness of thesystem.

[0121] Measurements were taken during different times of the day andnight, and at several receiver sites.

[0122] Measurement 2:

[0123] Measurement of the Interference Produced by the ExistingMicrowave Transmitters on the PCN

[0124] The position of the four vans shown in FIG. 16 varied relative tothe existing microwave transmitters to determine the sensitivity of thePCN to such interference. Both qualitative voice measurements andquantitative bit-error-rate measurements were made.

[0125] The robustness of the system to fading also was determined. Thismeasurement of the effect of the propagation characteristics of thechannel on the PCN were made by positioning the cell and mobile users indifferent parts of Orlando. BER measurements were taken, and acomparison to r², r^(3.6 and r) ⁴ curves were made in an attempt tobetter characterize this PCN channel. FIGS. 17A-17K plot attenuationversus distance based on these experimental results.

[0126] Fading Due to Multipath

[0127] The received waveform often includes numerous similar signalseach delayed with respect to one another. This delay is due to the factthat the antenna transmits the same signal, with equal power, in alldirections simultaneously. Some of these signals, after bouncing off ofcars, buildings, roadways, trees, people, etc., are received after beingdelayed. These are called multipath signals. Thus, the total receivedsignal is:${v_{R}(t)} = {\sum\limits_{i = 1}^{N}\quad {{a_{i}\left\lbrack {{d_{i}\left( {t - \tau_{i}} \right)} \oplus {g_{i}\left( {t - \tau_{i}} \right)}} \right\rbrack}\cos \quad {w_{o}\left( {t - \tau_{i}} \right)}}}$

[0128] where d_(i) is the data, g_(i) is the pseudo-noise (PN) sequenceand ⊕ indicates modulo-2 addition.

[0129] If several τ₁ are clustered together so that the differencebetween the largest τ_(i) =τ_(k) and the smallest τ_(i)=τ₁, is less thanthe duration of a chip, i.e., τ_(k)−τ₁<T_(c), then the received signalV_(R) (t) can be severely attenuated. This is called “fading” due tomultipath.

[0130]FIG. 18 shows the spectrum of a 24 Mchips/s direct sequence spreadspectrum signal at a carrier frequency of 1.956 GHz when multipathfading is present. Note that a 8 dB deep, 15 MHz wide fade can result.Other experiments performed indicate that typical fades are 10 dB orgreater and 1-3 MHz or more wide. Thus, a 48 MHz bandwidth, widebandspread spectrum signal is relatively insensitive to muiltipath fades,while “narrowband” signals having bandwidths of less than 3 MHz can begreatly attenuated due to fading.

[0131] Based on these findings personal communication networks accordingto the present invention using CDMA have numerous advantages as comparedto FDMA and TDMA.

[0132] They can be used in a frequency band that has existing users, andtherefore this means of communication represents an effective, efficientmode of frequency band utilization.

[0133] Broadband-CDMA modulation is more robust in the presence ofmultipath. For example, if the direct path is 600 feet and the multipathis 800 feet, the two returns are separated by 200 feet or 200 ns. Usingbroadband-CMDA modulation the chip rate of 25 Mchips/s means that thetwo returns are uncorrelated. Indeed, multipath returns exceeding 40feet are uncorrelated and do not result in fading.

[0134] CDMA has the potential of allowing a larger number of users, thatis, of being a more efficient system than either TDMA or FDMA. Thisimprovement can also be translated into lower power and hence longerlife for batteries.

[0135] In this decade, the CDMA PCN system is likely to be widely usedfor voice communications, facsimile transmission and other types of datatransmission. Its versatility could well result in this system attaininga major share of the world's communication market.

[0136] It will be apparent to those skilled in the art that variousmodifications can be made to the spread spectrum CDMA communicationssystem using the notch filter of the instant invention without departingfrom the scope or spirit of the invention, and it is intended that thepresent invention cover modifications and variations of the spreadspectrum CDMA communications system using the notch filter provided theycome in the scope of the appended claims and their equivalents.

What is claimed is:
 1. A spread spectrum base station comprising: meansfor generating a plurality of spread spectrum signals, the spreadspectrum signals encompassing a selected frequency spectrum; means fordetecting frequencies within the selected frequency spectrum having ahigh microwave power; means for notch filtering the spread spectrumsignals so that a transmitted version of the spread spectrum signals isnotch filtered at the detected frequencies; and means for transmittingthe notch filtered spread spectrum signals.
 2. The base station of claim1 wherein the notch filtering is performed at intermediate frequency. 3.The base station of claim 1 wherein the notch filtering is performed atradio frequency.
 4. A spread spectrum base station comprising: aplurality of mixers for mixing data signals with codes to generate aplurality of spread spectrum signals; a sensor for detecting frequencieswithin a selected frequency spectrum having a high microwave power; aplurality of notched filters for notch filtering the spread spectrumsignals so that a transmitted version of the spread spectrum signals isnotch filtered at the detected frequencies; and an antenna fortransmitting the notch filtered spread spectrum signals.
 5. The basestation of claim 4 wherein the notch filtering is performed atintermediate frequency.
 6. The base station of claim 4 wherein the notchfiltering is performed at radio frequency.